What is the tablighi jamaat?

The tablighi jamaat was founded by Hadhrat Moulana Muhammad Ilyaas Sahib Kandhelwi (rahmatullahi alaih). Moulana had lived for a long time in Nizaamuddeen, Delhi. The Muslims in Mewat, an area close to Delhi were very distant from Islam; they practised various Hindu customs and were ignorant of the performance of namaaz and the kalimah etc. Hence, Moulana had initially commenced with tabligh among them and he created within them the fervour and enthusiasm for learning deen. Subsequently, he would take groups of men, ten to twelve at a time and join them with someone who knew how to perform namaaz and wudhu and recite the Qur'aan and from him they would learn the same. They would then go to different areas encouraging the residents of those areas to join them in this mission. In this way they would spend 40 days in the path of Allah, during which time they refrained from their bad and evil habits of drinking, zina, swearing, and quarrelling and fighting, etc. Upon their return, they would encourage others from their village to also spend a period of 40 days in this manner. Through this, they had gained a lot of benefit and the Muslims at large also gained great benefit. This in a nutshell is called tabligh and those involved in this effort and mission are referred to as the tabligh jamaat.

And Allah Ta'ala (الله تعالى) knows best.